Why is there no minimum intelligence score required to be a magic user?
In most of the B/X and AD&D clones, in order to be a dwarf, halfling, monk, paladin, druid, assassin and even an elf, you need minimum score in one or two abilities. But anyone can be a magic user.
If I wanted to create a low magic campaign or a campaign where magic was rare and dangerous, as in Jack Vance’s work, I’d require an intelligence of 14 or better. That would still allow about 30 percent of the population to learn magic spells. If you bump it to 15, about 11 percent would qualify.
In a world like that, magic would be something to fear, something that is exceedingly rare.